The present invention relates to a transparent protective substrate for use in a glare reducing (reflection reducing) treatment of the surface of a polarizing plate used on the surface of a liquid crystal display and the like, a method for producing the same and a polarizing plate using the substrate.
A polarizing plate which serves as a light shutter is provided on the surface of a display such as a liquid crystal display. Since the polarizing plate, as such, has a poor marring resistance, it is protected by a transparent protective substrate such as a glass, a transparent plastic plate or a transparent plastic film. The transparent protective substrate comprising a transparent plastic plate, a transparent plastic film or the like is liable to flawing. For this reason, in recent years, a transparent protective substrate having a surface which was rendered mar-resistant has been developed. Such a technique is described in, for example, Japanese Patent Laid-Open Publication No. 105738/1989.
This laid-open publication discloses a transparent protective substrate to which marring resistance, chemical resistance and glare reducing property have been imparted, that is, a triacetate film for regulating light. Such a film is used for lamination on a polarizing film to constitute a polarizing plate. In this film, a curing coating comprising an ultraviolet curing epoxyacrylate resin is provided on one surface of an unsaponified triacetate film to form a triacetate film having excellent marring resistance and chemical resistance.
Further, in order to impart a glare shielding property to the above-described film, a resin composition comprising the above-described ultraviolet curing epoxyacrylate resin and an amorphous silica added thereto is coated on the surface of the triacetate film, and the resulting coating is then dried. In the lamination of the resultant triacetate film on a polarizing film to form a polarizing plate, the triacetate film is saponified with an alkali for the purpose of enhancing the adhesion between the triacetate film and a polarizing film and, at the same time, conducting destaticization, and then laminated on the polarizing film to provide a polarizing plate.
In the above-described conventional transparent protective substrate having a glare reducing property, however, a coating of a resin composition containing an amorphous silica is provided on the surface thereof. This coating is formed by coating the surface of the transparent substrate for the purpose of imparting a glare reducing property to the surface of the transparent protective substrate. In order to impart the glare reducing property, silica is incorporated in an amount of 2 parts by weight based on 100 parts by weight of the resin. The coating containing silica according to such formulation causes the transparency to be deteriorated. Further, in this case, when a triacetate film is used as the transparent substrate, the saponification of the silica-containing coating for the purpose of improving the adhesion and, at the same time, conducting destaticization increases the haze value of the resultant triacetate film, which leads to a deterioration in the resolution, contrast and transparency of the film. The haze value is a value expressed in terms of diffused transmission/total light transmission, that is, shows the proportion of diffused light to the whole transmitted light.